Shortly after the three riders enter the desert Hightower discovers that Pedro's water bag is empty and throws it away. As they ride away in a long shot, the water bag is still visible on Pedro's horse.
When the three Godfathers leave the covered wagon with little Robert William Pedro, the canvas is coming loose in the wind. Later when Purly Sweet and his posse arrive at the covered wagon, the canvas is tied down firmly.
The coach car on the train in Welcome has 7 windows, no roof-walk, and the number under the windows. The car at Mojave Tanks, which is supposed to be the same, has 5 windows, a roof-walk, and the number in front of the windows. The 5-window car also has no interior, as seen through the rear door.
When the train stops in town the tender is correct. When it is shown running in the desert the tender is incorrect. It is an oil tender and is incorrect for that time period. The fuel would have been wood.
In the chase scenes after the bank robbery, the shadows change location repeatedly, indicating the scenes were shot at different times of the day.
What appear to be power lines are visible overhead behind Ruby Latham when the stagecoach is stopped in Welcome, Arizona. They were intended to be interpreted as telegraph lines.
When Hightower is greasing the baby, who is referred to as a boy, is played by a girl.
Near the end of the movie, Hightower reads a bible passage Matthew, yet the bible is open close to the beginning.
When Ben Johnston is standing in the buckboard when the sheriff and 3 deputies use it to chase the 3 bank robbers out of town, he has one leg behind him bracing himself to stay standing while the horses gallop. There is a strap secured to the tray of the buckboard and he has the toe of his boot wedged in it to help keep his balance.
When Robert Hightower (John Wayne) stumbles into the New Jerusalem saloon, the people are signing "The Holy City." Although it is unclear when this movie was supposed to take place, the women's fashions indicate it is in the 1870s. "The Holy City" wasn't written until 1892 and didn't become popular until around the turn of the century (1900).
When Pedro is using a rattle you can hear it even when he stops shaking it.
The soon-to-be-a-mother in the wagon (played by Mildred Natwick) is described by Robert Hightower (John Wayne) as being "young, 28 or 30." Mildred Natwick was 43 when the movie was released in 1948.
While Wayne is holding the baby at the wagon, Pete is asked if he got any information about looking after the baby from its dying mother. He is supposed to say "Do you think I was going to drive the lady crazy?" what he says is "Do you think I was going to drive the lazy crazy?"